Prepare Crumb Topping: Preheat oven to 350°. Combine first 5 ingredients; cut cold butter into flour mixture with a pastry blender or fork until crumbly. Cover and chill until ready to use.
Prepare Cake Batter: Beat softened butter at medium speed with a heavy-duty electric stand mixer until creamy. Add 1 1/4 cups granulated sugar, beating 3 minutes or until fluffy. Add egg yolks, 1 at a time, beating just until yellow disappears.
Stir together flour and baking soda; add to butter mixture alternately with yogurt, beginning and ending with flour mixture. Stir in lemon zest.
Beat egg whites at high speed until foamy; gradually add remaining 1/4 cup granulated sugar, 1 Tbsp. at a time, beating until stiff peaks form. Fold into batter. Pour batter into a greased and floured 9-inch square (2-inch-deep) pan. Dollop with lemon curd; gently swirl with a knife. Sprinkle Crumb Topping over batter.
Bake at 350° for 45 to 50 minutes or until a wooden pick inserted in center comes out clean. Remove from oven; dust with powdered sugar. Serve warm, or cool on a wire rack 1 hour.
This was really good! Except I had trouble telling if it was completely cooked since the cake was on the bottom and the lemon curd was above it. I couldn't tell if the cake was under-cooked or if it was just the texture of the lemon curd I was tasting. The crumb topping was good though, and my toothpick came out clean, so I guess it was done? It tasted really good though, it was just really messy.